Method for producing a steel sheet or strip for making a can, and steel sheet or strip obtained by said process

ABSTRACT

Process for producing a sheet or strip for making a can obtained by drawing and ironing from a steel having the following composition in percentage by weight: Carbon less than 0.008%, Manganese between 0.10 and 0.30%, Nitrogen less than 0.006%, Aluminium between 0.01 and 0.06%, Phosphorus less than 0.015%, Sulphur less than 0.020%, Silicon less than 0.020%, a maximum of 0.08% of one or more elements selected from copper, nickel and chromium, the remainder being iron and residual impurities, in which process the slab is hot rolled into a hot sheet or strip having a thickness less than 3 mm, and then the hot sheet or strip is cold rolled with a reduction of between 83 and 92% and subjected to a recrystallization annealing and cold rerolled with a reduction of between 10 and 40%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing a steel sheetor strip for making a can obtained by drawing and ironing, of thebeverage can type.

The present invention also relates to a steel sheet or strip for makinga can obtained by drawing and ironing.

2. Description of the Background

This type of can usually comprises a bottom, a thin peripheral wall anda neck for achieving the setting or seaming of a lid, which may be ofthe easily-opened type, and is produced in particular by drawing andironing a cup cut from a metal sheet or strip.

For this purpose, the cup is subjected first of all to a drawingoperation with a relatively severe reduction on a press which comprisesin the conventional manner, on one hand, a fixed punch and a supportforming a peripheral blank holder which is slidable around said punchand on which the cup rests, and, on the other hand, a die to be appliedagainst the cup with a force transmitted vertically by an upper slide.

The cup, comprising a bottom and a flange formed by the drawingoperation, is then either calibrated by a light drawing operationwithout the use of a blank holder, or redrawn with a blank holder and isthen subjected to an ironing operation which comprises drawing theflange, by means of a draw die with successive reductions, so as toprogressively form the thin peripheral wall of the can.

Thereafter, the bottom is formed on the draw die so as to impart theretoa given geometry and the neck of the thin peripheral wall is formed inaccordance with two methods, namely a necking method with a die, nameddie-necking, or a necking method employing a forming roller, named spinnecking

The method for necking with a die comprises forcing the neck into a diehaving a conical inlet profile and a cylindrical outlet profile. Acylindrical element guides the formed wall as it leaves the die.

The force required to permit the deformation of the metal is derivedfrom the thrust applied on the bottom of the can and axially transmittedby its thin peripheral wall.

To reach the desired inside diameter, a plurality of successivereductions are required, each being a distinct forming step. When thereduction in diameter is obtained, flanging is usually effected withflanging rollers.

The spin-necking method employing a forming roller comprises driving thecan in rotation while it is maintained between a pusher and a centringring.

The free end of the thin peripheral wall is engaged on a mandrel and twoaxially travelling rollers form the neck of the can which progressivelyleaves the mandrel while always being maintained between the pusher andthe centring ring.

The profile of the neck is obtained by the simultaneous displacements ofthe rollers, the centring ring and the pusher.

Subsequent to these various operations, the can is filled and a lid, forexample of the easily-opened type, is set or seamed on the neck of thecan.

It is known to use for making this type of can a steel sheet or strip ofextra-soft steel type the composition of which in percentage by weightis the following:

Carbon of the order of 0.030 to 0.040%

Manganese of the order of 0.15 to 0.25%

Nitrogen of the order of 0.004 to 0.006%

Aluminium of the order of 0.03 to 0.05%

Phosphorus less than 0.015%

Sulphur less than 0.020%

Silicon less than 0.020%,

a maximum of 0.08% of one or more elements selected from copper, nickeland chromium, the remainder being iron and residual impurities.

The sheet or the strip is produced by a process in which the slabissuing from a continuous casting operation is hot rolled, then coldrolled to obtain a thin sheet or foil which is subjected to arecrystallization annealing operation at a temperature below Ac1.

This process permits obtaining a thin sheet or foil which has a finalthickness of about 0.30 mm, and making from this sheet a can whose thinperipheral wall has, after drawing and ironing, a thickness of the orderof 0.1 mm.

Now, manufacturers of cans, for reasons of economy and increasedproductivity, aim at producing cans of reduced weight, i.e. with thinnerwalls.

In order to enable the can with thinner walls to withstand the pressureof the liquids it contains, particularly when it concerns a gaseousbeverage, and in order to ensure that the can itself has a sufficientstrength, steels of improved mechanical characteristics must be used.

In order to improve the mechanical characteristics, manufacturing firmshave, with the use of an extra-soft steel of the aforementionedcomposition, subjected a slab to a hot rolling and to a cold rolling toobtain a sheet which is subjected to a recrystallization annealing at atemperature below Ac1, and is then cold rerolled.

But it is known that a reduction in the thickness or an improvement inthe mechanical characteristics of sheets or strips accentuates thephenomena of creasing when making the cans.

Tests have shown that this process results in a narrowing of the rangeof drawability of the sheet or the strip and an increase in the earingcoefficient.

A narrower drawability range results in difficulties in the forming ofthe bottom and is the origin of the appearance of wrinkles during thedrawing operation.

In order to avoid the formation of wrinkles when drawing, the pressureexerted by the blank holder on the sheet blank may be increased, butthis increase in the pressure of the blank holder creates a problem ofthe control of the flow of the metal during the drawing and mayconsequently cause the metal to fracture or tear, particularly in theregion of the connection or corner radii.

Further, the increase in the earing coefficient creates a problem whenremoving the can from the draw punch, i.e. during the strippingoperation.

Indeed, this operation is carried out by sliding a ring along the drawpunch so that it can bear against the free edge of the thin peripheralwall of the can.

When the thin peripheral wall of the body of the can has large earings,the stripping ring only bears against a few points of the peripheralwall and very often there occurs a creasing of the peripheral wallduring the stripping and the can must be scrapped.

In order to reduce the earing coefficient, it is known to coil up thesheet in the hot state before the cold rolling and recrystallizationannealing.

But this additional operation results in drawbacks since the edges ofthe sheet or strip are in direct contact with the surrounding air andcool quicker than the centre part.

This natural cooling differential between the edges and the centreresults in a heterogeneity of the mechanical characteristics of thesheet or strip. Moreover, the coiling up of the sheet in the hot stateresults in the formation of a coarse cementite.

The coarse cementite may result in the piercing of the thin peripheralwall when forming the neck and a tearing away of this metal during thedrawing operation owing to hard particles in the steel.

Further, the presence of hard particles in the steel results in apremature wear of the various drawing and ironing tools.

Consequently, manufacturers are faced with serious problems which areoften antinomic when they attempt to reduce the thickness of the wallsof the cans.

SUMMARY OF THE INVENTION

An object of the invention is to avoid these drawbacks by providing aprocess for producing a sheet or strip for making a can obtained bydrawing and ironing which permits reducing the thickness of the walls ofthe can and consequently achieving a saving in weight.

The invention provides a process for producing a sheet or strip formaking a can obtained by drawing and ironing, of the beverage can type,from a steel having the following composition in percentage by weight:

Carbon less than 0.008%

Manganese between 0.10 and 0.30%

Nitrogen less than 0.006%

Aluminium between 0.01 and 0.06%

Phosphorus less than 0.015%

Sulphur less than 0.020%

Silicon less than 0.020%

a maximum of 0.08% of one or more of the elements selected from copper,nickel and chromium, the remainder being iron and residual impurities,in which process the slab is hot rolled into a hot sheet or band havinga thickness of less than 3 mm, then the hot sheet or the band is coldrolled with a reduction of between 83 and 92% and subjected to arecrystallization annealing at a temperature lower than Ac1 and finallycold rerolled with a reduction of between 10 and 40%.

The invention also provides a steel sheet or strip for making a canobtained by drawing and ironing, of the beverage can type, characterizedin that it is obtained by the aforementioned process.

DETAILED DESCRIPTION OF THE INVENTION

A better understanding of the invention will be had from the followingdescription given solely by way of example.

The manufacturer of a can, of the beverage can type, by drawing andironing comprises cutting a blank from a steel sheet or strip, thendrawing this blank with a relatively severe reduction to form a cup.

Thereafter, the cup comprising a bottom and a flange is calibrated andsubjected to an ironing comprising drawing the flange with successivereductions to form progressively the peripheral wall of the can.

The bottom is then formed so as to impart thereto the given geometry andthe neck of the thin peripheral wall is formed either by a neckingmethod with a die, the die necking method, or by a necking methodemploying a forming roller, the spin-necking method.

In order to be able to manufacture a can having very thin walls, theinvention proposes producing this type of can by said drawing andironing operation from a very low carbon steel having the followingcomposition in percentage by weight:

Carbon less than 0.008%

Manganese between 0.10 and 0.30%

Nitrogen less than 0.006%

Aluminium between 0.01 and 0.06%

Phosphorus less than 0.015%

Sulphur less than 0.020%

Silicon less than 0.020%

a maximum of 0.08% of one or more of the elements selected from thecopper, nickel and chromium, the remainder being iron and residualimpurities, in which process the slab is hot rolled into a hot sheet orstrip having a thickness of less than 3 mm, then the hot sheet or stripis cold rolled with a reduction of between 83 and 92% and subjected to arecrystallization annealing at a temperature below Ac1 and finally coldrerolled with a reduction of between 10 and 40%.

The slab is hot rolled into a sheet having a thickness of between 1.8and 2.5 mm and preferably between 2 and 2.4 mm, then the sheet is coldrolled with such reduction as to bring the sheet to a thickness ofbetween 0.26 and 0.32 mm and subjected to a recrystallization annealingat a temperature below Ac1 and lastly cold rerolled with a reduction ofbetween 28 and 35% to bring the sheet to a thickness of between 0.18 and0.22 mm.

The recrystallization annealing is a continuous annealing.

In order to be able to produce a thin steel sheet or strip having athickness between 0.18 and 0.22 mm and having all the characteristicsrequired for making cans, namely drawn and ironed cans the walls ofwhich have a thickness equal to and even less than 0.07 mm, it wasrealized that it is necessary to use a very low carbon steel having acarbon content lower, in percentage by weight, than 0.008% and toproduce this steel in accordance with the double reduction method, i.e.to subject the hot rolled sheet or strip to a cold rolling followed by arecrystallization annealing and a cold rerolling.

Surprisingly, it was realized that to achieve the optimum mechanicalcharacteristics to permit carrying out the drawing and ironing necessaryto obtain a can whose walls have a thickness equal to 0.07 mm, thereduction produced by the first cold rolling of the sheet or strip hadto be reduced.

Indeed, for example if there is examined the earing coefficient of asteel sheet or strip which was produced from a steel having thefollowing composition in percentage by weight:

Carbon 0.003%

Manganese 0.204%

Phosphorus 0.009%

Sulphur 0.009%

Nitrogen 0.003%

Silicon 0.002%

Copper 0.008%

Nickel 0.021%

Chromium 0.017%

Aluminium 0.027%

the remainder being iron, and which was hot rolled to obtain a hotrolled strip 2.3 mm thick, then cold rolled to obtain a strip 0.26 mmthick, and continously annealed at a temperature below Ac1 and finallycold rerolled to bring this strip to a thickness of 0.18 mm, the earingcoefficient is equal to -0.2.

On the other hand, a sheet or strip which was produced from the samesteel but hot rolled to bring it to a thickness of 1.8 mm then coldrolled to obtain a strip 0.26 mm thick, then continuously annealed underthe same conditions and cold rerolled to bring it to a thickness of 0.18mm, has a earing rate equal to -0.05, which is a coefficient very closeto 0 and therefore represents a steel having a very low tendency to formearings.

It is therefore particularly important to conform to the rates of coldrolling and rerolling after annealing and to apply a high hot rollingrate so as to produce a hot rolled strip having a thickness of less than3 mm and preferably between 1.8 and 2.5 mm.

Apart from this aspect concerning the process for obtaining the strip,it is also necessary to use a steel having a very low carbon content tobe able to produce very thin, drawn and ironed cans.

In the following table 1, different steel compositions are indicated,the steels A to F being very low carbon steels, i.e. steels having acarbon percentage lower than 0.006%, and the steels G and H extra-mildsteels.

                                      TABLE 1                                     __________________________________________________________________________    STEELS                                                                             C   Mn P  S  N  Si Cu Ni Cr Al                                           __________________________________________________________________________    A    0.0032                                                                            0.192                                                                            0.008                                                                            0.010                                                                            0.003                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.048                                        B    0.0029                                                                            0.192                                                                            0.008                                                                            0.011                                                                            0.005                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.047                                        C    0.0028                                                                            0.192                                                                            0.009                                                                            0.011                                                                            0.004                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.048                                        D    0.0027                                                                            0.192                                                                            0.009                                                                            0.012                                                                            0.003                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.047                                        E    0.0033                                                                            0.198                                                                            0.012                                                                            0.009                                                                            0.002                                                                            0.003                                                                            0.006                                                                            0.018                                                                            0.018                                                                            0.030                                        F    0.0030                                                                            0.204                                                                            0.009                                                                            0.009                                                                            0.003                                                                            0.002                                                                            0.008                                                                            0.021                                                                            0.017                                                                            0.027                                        G    0.0274                                                                            0.192                                                                            0.009                                                                            0.011                                                                            0.004                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.048                                        H    0.0282                                                                            0.192                                                                            0.009                                                                            0.012                                                                            0.003                                                                            0.007                                                                            0.007                                                                            0.019                                                                            0.015                                                                            0.047                                        __________________________________________________________________________

Slabs each having one of the compositions indicated in the foregoingtable 1, were subjected to a treatment comprising hot rolling each slabinto a sheet then cold rolling this sheet and subjecting it to arecrystallization annealing at a temperature below Ac1, and finally acold rerolling.

The steel sheets or strips obtained by this process were subjected totests in order to determine the yield strengths Y.S and the ultimatetensile strengths U.T.S in the direction of the length and in thetransverse direction, and the earing coefficient ΔC.

The results are indicated in the following table 2.

                                      TABLE 2                                     __________________________________________________________________________    Cold rolling                                                                             Cold rerolling                                                     rate       rate   Tension (length)                                                                           Tension (width)                                                                            ΔC                          ACIERS                                                                             (reduction)                                                                         (reduction)                                                                          Y.S (MPA)                                                                           U.T.S (MPA)                                                                          Y.S (MPA)                                                                           U.T.S (MPA)                                                                          aniso                             __________________________________________________________________________    A    88.7% 31%          595          625    -0.20                             B      85% 21%    509   512    462   554    -0.06                             C      88% 16%    457   475    467   503    -0.04                             D    90.7% 21%    513   517          555    -0.24                             E    90.4% 16%    463   475    487   506    -0.13                             F    91.1% 10%    384   400    458   418    -0.11                             G      86% 11%    455   477    360   501    -0.28                             H    84.3% 20%    532   551    350   584    -0.41                             __________________________________________________________________________

This table shows that the steels G and H, although they satisfy therolling conditions of the process according to the invention, have acoefficient ΔC further from 0 than the steels B, C, E.

Indeed, the steel B and the steel H were subjected to similar hotrolling, cold rolling, annealing and cold rerolling conditions. However,the steel H has higher yield strength and ultimate tensile strengthvalues and above all a very much lower ΔC which is much further from 0.

Likewise, although the steel G was subjected to a cold rolling rate of86% and a rerolling rate of 11% which are lower than those to whichsteel C was subjected, the ΔC of steel G is further from 0 than the ΔCof steel C.

Further, with the cold rolling rate of steel B at 85% and that of steelD at 90.7% and as these two steels were subjected to the same rerollingafter annealing, the ΔC aniso of steel D is 0.24 and the ΔC aniso ofsteel B is -0.06.

Therefore, the steel sheet or strip of very low carbon content lowerthan 0.008% produced by the process according to the invention, i.e.with a hot rolling, a cold rolling with a reduction of between 83 and92%, then a recrystallization annealing at a temperature below Ac1 andfinally a cold rerolling with a reduction of between 10 and 40%, has ayield strength in the direction of the length of between 350 and 450 MPafor a final thickness of about 0.22 mm, between 440 and 540 MPa for afinal thickness of about 0.20 mm and between 500 and 600 MPa for a finalthickness of about 0.18 mm.

The sheets or strips according to the invention may also becharacterized by the fact that the number of grains of ferrite per mm²is between 10000 and 30000 and preferably between 15000 and 25000, whichcorresponds to a very small grain size.

This is important for the regularity of the characteristics of the metalthroughout the length of the coil, and for avoiding the antinomicdrawbacks with regard to the drawing, the ironing and the forming of theneck.

The process for producing a sheet according to the invention alsopermits retaining a given quantity of carbon in solution in the sheet.

Such a sheet therefore has the characteristic of hardening in asignificant manner when stoving the varnish carried out on the can afterit has been put into shape.

This characteristic is very important in the case of the manufacture ofcans obtained by drawing and ironing since the sheet according to theprocess of the invention has adequate mechanical characteristics forfacilitating the forming of the can, the mechanical characteristicsvarying little with respect to time.

Once the can has been formed, varnished and subjected to the stoving ofthe varnish, the mechanical characteristics are significantly improved,which has the advantage of increasing the strength of the can.

This strength of the can is in particular characterized by the pressurefor inverting the dome of the bottom of the can.

This inverting pressure, which is the limit pressure beyond which thedome produced on the bottom of the can is inverted, increases by about10% after stoving and changes for example from 6.3 to 6.9 bars for agiven type of can.

This is particularly the case for the cold rerolling rate, afterannealing the sheet, of between 10 and 30%.

In this way, the process according to the invention for producing asteel sheet or strip having a very low carbon content for making a can,of the beverage can type, obtained by drawing and ironing, permitsreducing the thickness of the walls of the can and achieving a saving inweight of about 30% on the sheet or strip, while widening the range ofdrawability and reducing the earing coefficient and the risk of theformation of wrinkles when drawing the can.

What is claimed is:
 1. Process for producing a sheet or strip for makinga can obtained by drawing and ironing, from steel having the followingcomposition in percentage by weight;Carbon less than 0.008% Manganesebetween 0.10 and 0.30% Nitrogen less than 0.006% Aluminum between 0.01and 0.06% Phosphorus less than 0.015% Sulphur less than 0.020% Siliconless than 0.020% a maximum of 0.08% of one or more of the elementsselected from copper, nickel and chromium, the remainder being iron andresidual impurities, provided that the steel contains at least 0.0015%of C. which process comprises hot rolling a slab into a hot sheet orstrip having a thickness between 1.8 and 2.5 mm, cold rolling the hotsheet or the strip with a reduction to bring the sheet to a thicknessbetween 0.26 and 0.32 mm, subjecting the sheet to a recrystallizationannealing at a temperature lower than Ac1 and cold re-rolling with areduction of between 10 and 40%.
 2. Process according to claim 1,wherein the slab is hot rolled into a strip having a thickness ofbetween 2 and 2.4 mm.
 3. Process according to claim 1, wherein the stripis cold rerolled with a reduction of between 28 and 35%.
 4. Processaccording to claims 1, wherein the strip is cold rerolled with suchreduction as to bring said strip to a thickness of between 0.18 and 0.22mm.
 5. Process according to claim 1, wherein the recrystallizationannealing is a continuous annealing.
 6. Steel sheet or strip for makinga can obtained by drawing and ironing, characterized in that it isobtained by the process according to claim
 1. 7. Steel sheet or stripaccording to claim 6, having a yield strength in the direction of thelength between 350 and 450 MPa for a sheet or strip having a finalthickness of about 0.22 mm, between 440 and 540 MPa for a sheet or striphaving a final thickness of about 0.20 mm, and between 500 and 600 MPafor a sheet or strip having a final thickness of about 0.18 mm.
 8. Steelsheet or strip according to claim 6, wherein the number of grains offerrite per mm² is between 10,000 and 30,000.
 9. A method of making abeverage can comprising drawing and ironing the steel sheet or strip ofclaim
 6. 10. Steel sheet or strip according to claim 6, wherein thenumber of grains of ferrite per mm² is between 15,000 and 25,000.
 11. Aprocess according to claim 1 wherein the carbon content is at least0.0027 wt. %.